1. Field of the Invention
The present invention relates to a magnetic head using a specific sealing glass for assembling.
2. Description of the Related Art
With the progress of electronics, need for various kinds of glass has been increased.
Conventional sealing glasses will be described by making reference to a sealing glass for magnetic heads.
FIG. 1 shows a perspective view of a typical magnetic head for a VTR which comprises a core made of ferrite. Ferrite core parts 1 are facing to each other with a constant gap distance therebetween, and a formed gap is filled with a gap glass 2 and bonded with a sealing glass 3. Around the ferrite core parts, a coil 4 is wound.
Although, the sealing glass should be sufficiently softened and flowed to bond the cores when heated, a softening or working point is preferably as low as possible to prevent interaction between the core material and the glass.
Further, the sealing glass has a coefficient of thermal expansion close to that of the ferrite. In general, the ferrite has a coefficient of thermal expansion of 80.times.10.sup.-7 to 115.times.10.sup.-7 /.degree.C. Recently, a ferrite having high saturation magnetic flux density has been developed to be used with a high resolution tape. Such new ferrite has a coefficient of thermal expansion of about 130.times.10.sup.-7 /.degree.C. (see, for example, Horikawa et al, the Preprint for the Society of Applied Physics, 4a-ZE-8, October, 1985). As magnetic materials as substitutes for the ferrite in the same fields, amorphous alloys, superstructured nitride alloys (see, for example, the Technical Study Reports of the Electronics, Information and Communication Society, MR-86-4, 87-14) and Sendust have been used. Among them, Sendust has a coefficient of thermal expansion of about 140.times.10.sup.-7 /.degree.C. Such new magnetic materials 5 are formed near the gap of the magnetic head by sputtering as shown in FIG. 2, in which "6" represents a gap depth, and " 16" represents an opening for winding. The sealing glass should have a coefficient of thermal expansion suitable for such structure of the magnetic head of FIG. 2.
The magnetic head is polished during processing. Since water is used in the polishing step, the sealing glass is required to have good water resistance. In addition, if the glass is darkly colored or devitrified (or crystallized) during processing, an apex 7 is not seen when the gap depth 6 is adjusted at a predetermined depth.
In view of the above facts, the sealing glass for the magnetic head should meet at least following requirements:
(a) Its working temperature is low.
(b) Its coefficient of thermal expansion is close to that of the magnetic material, for example, from 80.times.10.sup.-7 to 140.times.10.sup.-7 /.degree.C.
(c) It has good water resistance.
(d) It is not darkly colored and not devitrified (crystallized) during processing.
As a result of preliminary experiments on the water resistance of the glass, it was found that the glass preferably has water resistance of "First Class" when graded according to the classes determined by the Japan Optical Glass Industries Association (Nippon Kogaku Garasu Kogyokai), which is explained below.
From the above view points, various sealing glasses have been proposed.
(1) Japanese Patent Kokai Publication Nos. 255643/1985, 36135/1986 and 111935/1986 disclose a glass having a coefficient of thermal expansion of 70.times.10.sup.-7 to 130.times.10.sup.-7 /.degree.C. and a working temperature of from 400.degree. to 600.degree. C.
(2) Japanese Patent Kokai Publication No. 36040/ 1987 discloses a glass having a coefficient of thermal expansion of 112.times.10.sup.-7 to 155.times.10.sup.-7 /.degree.C. and a working temperature of from 400.degree. to 560.degree. C.
(3) Japanese Patent Kokai Publication No. 170240/1988 discloses a glass having a coefficient of thermal expansion of 119.times.10.sup.-7 to 126.times.10.sup.-7 /.degree.C. and a working temperature of 550.degree. C.
(4) Japanese Patent Kokai Publication No. 206330/1988 discloses a glass having a coefficient of thermal expansion of 85.times.10.sup.-7 to 105.times.10.sup.-7 /.degree.C. and a working temperature of from 500.degree. to 550.degree. C.
(5) One of commercially available glass, namely T 015 (manufactured by Iwaki Glass Co., Ltd.) has a coefficient of thermal expansion of 112.times.10.sup.-7 and a working temperature of 450.degree. C., and another, namely T 176 (manufactured by Iwaki Glass Co., Ltd.) has a coefficient of thermal expansion of 120.times.10.sup.-7 /.degree.C. and a working temperature of 430.degree. C.
However, each of the above conventional glasses has its own defects.
For example, since the glass (1) contains 30 to 70% of V.sub.2 O.sub.5, it is black. Therefore, this glass does not satisfy the above requirement (d).
Since the glass (2) contains a large amount of ZnO, a combination of SiO.sub.2 +SnO.sub.2 +TiO.sub.2 +ZrO.sub.2, and a combination of WO.sub.3 +MoO.sub.3 which crystallize the glass easily, raw materials are not vitrified during melting, or the formed glass is easily devitrified during processing. Then, this glass does not satisfy the requirement (d).
Since the glass (3) contains a large amount, for example, 5% or more of Na.sub.2 O, it has insufficient water resistance and does not satisfy the requirement (c).
Since the glass (4) contains a large amount of SiO.sub.2 or Al.sub.2 O.sub.3 which crystallizes the glass easily, it is easily devitrified during processing and does not satisfy the requirement (c).
Since the commercially available glasses (5) both contain a large amount of PbO and B.sub.2 O.sub.3 (80% or larger and 10% or larger, respectively), they have poor water resistance and do not satisfy the above requirement (c).